ML19261D766
| ML19261D766 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 05/25/1979 |
| From: | PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | |
| Shared Package | |
| ML19261D759 | List: |
| References | |
| DRF-B13-109-40, OPE-21-579, NUDOCS 7906260341 | |
| Download: ML19261D766 (10) | |
Text
e OPE-21-579 DRF-B13-109-40 FEECWATER N0ZZLE INSPECTION RECOPp.ENDATION PEACH BOTTOM UNIT 3
.~
PREPARED FOR PHILADELPHIA ELECTRIC CO.
BY GENERAL ELECTRIC COMPANY YAY 25,1979 2311 112 7 006260 3 y/
/
4 IMPORTANT NOTICE REGARDING CONTENTS OF THIS REPORT PLEASE READ CAREFULLY The only undertakings of General Electric Company with respect to information in this document are contained in the contract between Philadelphia Electric Company (PECO) and General Electric Company (reference Quotation Letter G-HE-8-254, December 27, 1978, and Purchase No. G-N 363438, January 29, 1979, and nothing contained in this document shall be construed as changing the contract. The use of this information by anyone other than PEC0, or for any purpose other than that for which it is intended, is not authorized; and with respect to any unauthorized use, General Electric Company makes no representation or warranty, and assumes no liability as to the completeness, accuracy, or usefulness of the information contained in this document.
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1.0
SUMMARY
The applicability of the examination requirements of SIL-207 to the Peach Bottom-3 feedwater nozzles were examined in light of the PB-3 unique operating history and previous inspections and repairs.
It was concluded that both the PT and UT examinations may be deferred from the Fall 1979 refueling outage to the end of fuel cycle number four.
2.0 BACKGROUND
During the first refueling outage (1977) for Unit No. 3, an external ultrasonic (UT) examination was performed on all six feedwater nozzles.
Ultrasonic reflectors requiring further investigation were found on two of the six nozzles (the "0" and "F" nozzles).
The feedwater spargers associated with these two nozzles were removed, the nozzle surfaces were flapper wheel cleaned, and liquid dye-penetrant (PT) examinations were performed. The PT examinations showed four (4) minor indications on the "F" nozzle.
No indications were present on the "D" nozzle.
The indica-tions on the "F" nozzle were removed by light grinding and did not penetrate the cladding into the base material.
Visual examination of both nozzles showed evidence that a metal-to-metal fit was still maintained at the time of sparger removal (a full circumferential band of unoxidized metal was visible in both safe ends).
Both spargers were reinstalled with an interference fit per the recommendations of the General Electric Company.
UT examination subsequent to the removal of the surface indications showed that the reflectors had not been removed and their characteristics were not changed. The consultant, Southwest Research Institute, in concurrence with General Electric, stated that most likely these indications were in the clad bond interface.
These feedwater nozzles were inspected ultrasonically again during the 1978 refueling outage.
No reportable indications were noted.
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It should be noted that actions have been taken at Peach Bottom to mitigate the feedwater nozzle cracking.
The startuo procedures were revised in 1977 so as to provide increased turbine bypass flow to the condenser during vessel heatup thus providing continuous feedwater flow during the startup interval. An automatic level control system has been installed (during the 1977 refueling outage) to position the three inch bypass valve which controlf condensate delivery to the reactor vessel prior to startup of the turbine-driven reactor feed pumps.
As pressure increases and the bypass valve approaches full open, a reactor feed pump is placed in service with its speed being automatically controlled via reactor level and its discharge valve throttled via operating procedure.
The bypass valve control systen and the feed pump operating procedure has eliminated batch feeding of condensate and has provided continuous feedwater flow thus minimizing the number and magnitude of feedwater temperature cycles.
In addition, a low flow feedwater control valve (approximately 0-8% feedwater flow), per GE SIL 208, will be installed around one af the reactor feed pump discharge valves.
The installation is planned for the Fall 1979 outage.
This valve will provide feedwater flow control between the control provided by the three inch reactor feed pump bypass valve which controls condensate delivery (referenced above) well on into the range of reactor feed pump speed control.
3.0 FNIER RECOMMENDATION AND BASIS The FNIER recommendation for nozzles that have been grind-repaired once is for UT examinations at each refueling outage and PT exams every 24 months or after every 20 startup/ shutdown cycles, whichever occurs earlier. The basis for this recommendation is presented in Section 7.1 of NED0-21821.
The major assumptions in this recommendation are:
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o Twelve month fuel cycle (Fuel Cycle 4 at PB-3 will be 18 months).
o Ten startup/ shutdown cycles per year (the projected numbers are eight/ year for Fuel Cycles 3 and 4, see Table 1).
o 0.50 inch deep grind depth removing previous crack (the maximum grind depth in a PB-3 nozzle was less -han the 3/16 clad thickness).
o 0.015 inch deep crack tip left after grinding.
o Crack growth analyses based on upper bound thermal transient stress.
Clearly, many of the assumptions made in the generic FNIER development are not applicable to Peach Bottom-3, and a new basis and recommendation is justified.
4.0 DEVELOPMENT OF INSPECTION RECOMMENDATIONS It is known that there were no detectable cracks in the feedwater nozzles after the examinations in 1978.
The objective is to determine what in-spections, if any, are required in 1979 to insure that there are no unacceptable cracks prior to the end of Fuel Cycle 4.
An unacceptable crack is one that would exceed the allowable flaw size as established by Section XI of the ASME Code.
This is found to be 0.71 inch total depth in the nozzle bore region.
4.1 Allowable Crack Death The basic requirement of Section XI of the ASME Code is to never permit a flaw to exist that is greater than one-tenth of the critical flaw size.
It is shown in Section 9.2 of NED0-21821 that the allowable flaw size may be conservatively taken as one-tenth of the wall thickness in a feed-water nozzle. The numerical valves are 0.95 inch in the blend radius and 0.71 inch in the nozzle bore for the allowable depths (the Peach Bottom-2 nozzle is geometrically identical to the example used in NED0-21821).
These are total crack depths that include the clad thickness plus the depth of the crack in the base metal.
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Crack growth studies in NED0-21821 show that faster crack growth occurs.
in the nozzle bore region.
The bore region also has a smaller allowable crack depth, so it is limiting.
The cladding has been machined out of the feedwater nozzle in a sister vessel to Peach Bottom-3.
The stress report (Reference 3) for this modified nozzle was examined to see if a 0.71 inch deep crack could be accommodated without weld repair.
It was found that after the nozzle bore is increased 1.38 inch after clad removal, that 27.1 in2 of surplus reinforcement area remains which may be applied to additional grind-outs.
(This reinforcement is based on the Alternate Rules for Nozzle Design, Paragraph NS-3339, in the 1974 and later editions of Section III, ASME B&PV Code and Code Case 1804.)
For Peach Bottom-3, 0.69 inch of a 0.71 inch bore crack could be removed with clad removal machinery.
The additional 0.02 inch could be removed by a grind-repair without using a 2
significant amount of the 27.1 in surplus reinforcement area.
There fore,
the 0.71 inch deep crack (established by taking one-tenth of the wall thickness) may be repaired by a combination of machining and grinding without the need for a weld repair.
4.2 Assumotions and Discussions It is assumed that there were cracks in the nozzles that were 0.44 inches deep after the UT exam in 1978. The UT examinations were performed by Southwest Research Institute who have the capability to detect cracks that are 1/4 inch deep in base metal, plus the 3/16 inch clad thickness, or a total depth of 0.44.
Based on the 1977 PT results, there are probably some cracks in the cladding.
It is not known how deep the cracks are, but the UT axam in 1978 showed they were less than 0.44 inch total depth.
For conservatism, the upper bound depth of 0.44 is assumed.
Following the conclusions about the initiation and propagation of feed-water nozzle cracks in NED0-21821, crack propagation from 0.44 inch to the allowable value of 0.71 inch is by startup/ shutdown cycles. The utility has recorded 6-1/2 cycles from the 1978 UT examinaticn to March, 1979; another 4-1/2 are projected fro the remainder of fuel cycle three, and 12 cycles are projected for fuel cycle four.
This totals to 23 cycles.
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It is calculated that it will take 26 cycles to propagate a crack from 0.44 inch to 0.71 inch.
This value is taken from Figure 4-139 of NEDO-21821 using the U = 500 curve. Normally, the U = 1000 or V = 2000 curve would be used for crack growth predictions since these curves correlate with the upper bound of field data.
However, the modified operating procedures used at PB-3 eliminate the large transient thermal stress on the feedwater nozzle that has been present at other plants during plant startup. Also the therma-1 sleeve inspection in 1977 found evidence that an interference fit existed between the thermal sleeve and safe end.
If the tight fit lasts through fuel cycle 4 and if the thermal sleeve to safe end joint does not leak, it would be reasonable to use the U = 100 curve irrespective of the startup procedure used by the utility.
It is concluded, therefore, that a reasonable and conservative crack growth curve to use is the U = 500 curve.
4.3 Conclusion Since the maximum possible depth of an undetected crack after the UT exam in 1978 is 0.44 inches, and it would take 26 startup/ shutdown cycles to propagate a 0.44 inch deep crack to the ASME Code allowable of 0.71 inch, and since only 23 cycles are projected to occur between the 1978 UT exam and the end of fuel cycle four, it is concluded that it is not necessary to perform any examinations of the feedwater nozzles at the end of fuel cycle three.
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5.0 REFERENCES
1.
Letter, W. A. Alden to J. Z. Sherk, dated March 20, 1979,
Subject:
Peach Bottom Unit 3 Feedwater Nozzle, Fall 1979, Inspection (also attached to GP-HE-9-19, March 26, 1979) 2.
NED0-21821, Boiling Water Reactor Feedwater Nozzle /Sparger Final Program Report, H. Watanabe, March, 1978 3.
22A5562 Rev. O, Reactor Vessel Stress Report, D. L. Rennels and M. A. Ross, October 5,1977.
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TABLE 1 OPERATING DATA SUPPLIED BY PEC0 (From Reference 1)
Time Startup to End of Fuel Cycle 1 (1977) 41 Start of Fuel Cycle 2 to End of Fuel Cycle 2 (1978) 12 Start of Fuel Cycle 3 to March,1979 5-1/2 March, 1979 to End of Fuel Cycle 3 (1979) 4-1/2 (Projected)
Start of Fuel Cycle 4 to End of Fuel Cycle 4 12 (Projected) 2311 120 9
MEC-21821 1.5
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NINE481LE POINT 1.4 1.3 1.2 y. 2GM*
U = 1000 3,i OVE RSE AS 8vvR A 1.0 U - 500 0.9 5
,I 0.8 A
~c PILGRIM g
y 0.7 mu 0.6 0.5 1
U *100 0.4 t
i 0.3 t
0.2
}
? Q ly.'Sb -->~
i O,1 I
I 8,
,,,.,,2 0
20 40 60 80 100 120 4
'.I NUMBE R OF STARTUP/ SHUTDOWN cycies t
i
- j Figure 4-139.
Feedwater Nozzle Crack Growth for Various Overall Heat
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Transfer Coefficients - Clad Nozzle 1
23ii i2i 4-280 4,
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-